1. Field of the Invention.
The present invention relates to switch devices for switching control of active elements from one parameter to another without causing disruptions or discontinuities in the control output devices.
2. Description of the Prior Art.
In the prior art, so called "bumpless transfer" has been sought after and various devices have been used for providing a transfer from one parameter to another. An example is U.S. Pat. No. 3,742,324 issued to Alan E. Gross et al. on June 26, 1973. This is a switching system for a servo mechanism that permits switching from one parameter to another, where the command is changed simultaneously with the feedback switch to match the forward loop error at the time of the switching. This technique works well, but involves substantial circuitry, and in active elements there is a possibility that a smooth transfer is not achieved when the variables are rapidly changing.
In particular, it has long been known to switch between "force" and "displacement" control in servo mechanisms.
In a servovalve controlled hydraulic press, and in particular in sheet molding compound presses, it is desirable to control the displacement between the two press parts as they are moving toward each other, and during the mold closing mode, but during the curing and final stages it is desirable to use force control for controlling the pressure between the two parts. This is set forth in U.S. patent application Ser. No. 237,690, filed Feb. 24, 1981, now abandoned in favor of a continuation application Ser. No. 493,651, filed May 11, 1983, and entitled Hydraulic Press, and owned by the same assignee as the present application, wherein a typical control circuit for such a press is illustrated in FIG. 12, and a mode switch is schematically represented at 163 in this figure. The mode switch of the present invention may be used for that control function in a circuit such as that shown in FIG. 12 of Application Ser. No. 237,690.
Additionally, the type of the circuit which can be controlled and utilized at this mode switch of the present invention is shown in U.S. Pat. No. 3,800,588. This circuit is also used for controlling SMC presses as outlined in said earlier Application Ser. No. 237,690.
A device that shows displacement control in a press until the force exceeds a set value and then switching to force control as shown in U.S. Pat. No. 3,825,386 which was issued to Bello et al. on July 23, 1974.
The circuit used in U.S. Pat. No. 3,825,386, however, does not provide the smooth transfer of the present device particularly when the elements are quite active.